A Dual Functional Diketopyrrolopyrrole‐Based Conjugated Polymer as Single Component Semiconducting Photoresist by Appending Azide Groups in the Side Chains

Abstract Molecular systems that can function as photoresists are essential for the fabrication of flexible electronics through all‐photolithographic processes. Most of the reported molecular systems for photo‐patterning of polymeric semiconductors contain binary or multi‐components. In comparison, single component semiconducting photoresist is advantageous since it will circumvent the optimization of phase separation and ensure the patterned semiconducting thin films to be more uniform. In this paper, a single component semiconducting photoresist (PDPP4T‐N3) by incorporating azide groups into the branching alkyl chains of a diketopyrrolopyrrole‐based conjugated polymer is reported. The results reveal that i) the azide groups make the side chains to be photo‐cross‐linkable; ii) uniform patterns with size as small as 5 µm form under mild UV irradiation (365 nm, 85 mW cm−2) at ambient conditions; iii) such photo‐induced cross‐linking does not affect the inter‐chain packing; iv) benefiting from the single component feature, field‐effect transistors (FETs) with the individual patterned thin films display satisfactorily uniform performances with average charge mobility of 0.61 ± 0.10 cm2 V–1 s–1 and threshold voltage of 3.49 ± 1.43 V. These results offer a simple yet effective design strategy for high‐performance single component semiconducting photoresists, which hold great potentials for flexible electronics processed by all‐photolithography.

All chemicals were purchased from commercial suppliers and used without further purification unless otherwise specified. Compounds 1 and 2 were purchased from Woerjiming (Beijing) Technology Development Institute, and Jiujiang Design Optoelectronic Materials Co., Ltd., respectively. F 4 BDOPV-2T was purchased from Beijing Hwrkchemical Co., Ltd., with number-average molecular weight (M n ) of 16.8 kDa and dispersity (Ð) of 3.7. PDPP4T was synthesized according to the procedures reported in the literature S1 with number-average molecular weight (M n ) of 40.3 kDa and dispersity (Ð) of 1.4. All operations involving azide containing species were kept away from UV light. 1 H NMR, 13 C NMR spectra in solution and the solid state 13 C NMR spectrum were recorded at a Bruker AVANCE III 400 MHz spectrometer. High-temperature 1 H NMR, 13 C NMR spectra were recorded at a Bruker Avance III 500WB NMR Spectrometer. Matrix-Assisted Laser Desorption/Ionization Time of Flight (MALDI-TOF) mass spectra were collected on a Bruker Solarix 9.4T FT-ICR mass spectrometer. Elemental analyses were performed on a Thermo Flash Smart instrument. Gel permeation chromatography (GPC) was performed on a Agilent PL-GPC 220 instrument using o-dichlorobenzene as an eluent (1.0 mL/min) at 140 °C. Polystyrene was utilized as the calibration standard, and PDPP4T-N 3 was dissolved in o-dichlorobenzene and the concentration was 1 mg/mL. Thermogravimetric analysis (TGA) analyses were carried on a PerkinElmer TGA 8000 Thermogravimetric Analyzer instrument under N 2 at a heating rate of 10 °C/min from 50 °C to 550 °C. UV-vis-NIR absorption spectra were collected on a HITACHI UH4150 UV-Vis spectrophotometer. PDPP4T-N 3 was dissolved in CHCl 3 and the concentration was 5 mg/mL. Thin film was fabricated by spin coating on quartz substrates. Fourier transform infrared spectra were recorded on a Bruker TENSOR-27. Cyclic voltametric (CV) measurements were carried out in a three-electrode cell by using glassy carbon as the working electrode, Pt as auxiliary electrode, and Ag/AgCl (saturated aqueous solution of KCl) as reference electrode on a computer-controlled CHI660C instrument at room temperature; the scan rate was 100 mV/s, and n-Bu 4 NPF 6 (0.1 mol/L in acetonitrile) was used as the supporting electrolyte. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc + ) was measured under the same conditions. HOMO and LUMO energy levels of this conjugated polymer was estimated with the following equations: HOMO = -(E ox onset + 4.8) eV, LUMO = -(E red onset + 4.8) eV.
Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nano scope IIIa multimode atomic force microscope in tapping mode under ambient conditions.
Optical microscope images were recorded using a Leica-DM4M microscope. Twodimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted on a Xenocs-SAXS/WAXS system with X-ray wavelength of 1.5418 Å. The film samples were irradiated at a fixed angle of 0.2°. A DSX-UVP60 UV LED Curing System was used as light source for the photo-patterning, and the light intensity was detected by a CEL-NP2000 optical power meter.
OFET and inverter devices were fabricated on a commercial Si/SiO 2 /Au substrate purchased from First MEMS Co. Ltd. A heavily n-doped Si wafer with a SiO 2 layer of 300 nm served as the gate electrode and dielectric layer, respectively. The Ti (2 nm)/Au (28 nm) source-drain electrodes were sputtered and patterned by a lift-off technique. Before deposition of the organic semiconductor, the gate dielectrics were treated with octadecyltrichlorosilane (OTS) in a vacuum oven at 120 °C, forming an OTS self-assembled monolayer, according to the reported method. S2 Then, the substrates were washed with CHCl 3 , hexane and isopropanol, sequentially. Polymer thin films were spin coated on the substrate with a thickness of around 30 -50 nm. The devices were thermally annealed at 160 °C for 10 min before measurement (in air for hole mobility measurement, while in a glovebox filled with N 2 for inverter measurement). The devices were measured on a Keithley 4200 SCS semiconductor parameter analyzer at room temperature. The channel width was 1400 μm, and the channel length was 50 μm. The calculations of mobilities, threshold voltages, on-off current ratios were carried out according to our previous report. S2

Synthesis of compound 5
Compound 4 (2.14 g, 1.54 mmol) was dissolved in 30 mL of tetrahydrofuran, followed by the addition of hydrochloric acid (12 mmol/mL, 20 mL